1. Field of the Invention
This invention relates to a lathe, more particularly to a lathe, which is provided with a polygon machining device that is used to machine a workpiece, such as a nut or a headed bolt.
2. Description of the Related Art
Referring to FIG. 1, a conventional polygon machining device 10 is shown to include a machine body 11, a right gear box 12, a rail member 13, a cutter carrying plate 14, a left gear box 15, three cutters 16 (see FIG. 2), and a universal connecting rod 17. A spindle 121 and a driving shaft 122 are journalled on the machine body 11. A chuck 123 is mounted on an end of the spindle 121 for holding a workpiece 18 (see FIG. 2) to be machined. A cutter shaft 151 is driven by the left gear box 15. A driven shaft 152 is driven by the universal connecting rod 17, and is connected operably to the left gear box 15. The cutters 16 are fixed on a cutter mounting disc 153, which is connected fixedly on the cutter shaft 151. When the spindle 123 rotates, it activates the driving shaft 122 via the right gear box 12, thereby rotating the cutter shaft 151, the cutter mounting disc 153 and the cutters 16 through a transmission line of the universal connecting rod 17, the driven shaft 152 and the left gear box 15. The aforesaid device 10 suffers from the following disadvantages:
(1) When the position of the cutter carrying plate 14 is adjusted in an axial or radial direction of the spindle 121, the universal connecting rod 17 flexes, thereby resulting in unsmooth rotation of the spindle 121 and the cutter shaft 151. Furthermore, because the length of the universal connecting rod 17 is limited, and because the device 10 can only be used to machine a workpiece, which is shorter than the universal connecting rod 17, the application range of the device 10 is reduced. PA1 (2) To form a nut or a headed bolt, a workpiece has to be machined on the device 10 and a lathe for forming threads, thereby resulting in an increase in the machining costs, the machine-occupied space and the tool-changing time.
U.S. Pat. No. 5,490,307 disclosed an improved lathe, which is provided with a polygon machining device to overcome the above-mentioned problems. In the improved lathe, a spindle and a polygon machining cutter unit are driven respectively by different driving sources at different rotational-speed ratios, and have no mechanical transmission therebetween. As a result, when the feeding distance of the polygon machining cutter unit is relatively large or when the material of a workpiece to be machined is relatively hard, the feeding of the cutter unit is delayed, thereby resulting in failure to form an accurate polygon on the workpiece.